1. Field of the Invention
In one of its aspects, the present invention relates to an isocyanate-based polymer foam and to a process for production thereof. More particularly, the present invention relates to an isocyanate-based polymer foam, inter alia, having improved anti-yellowing properties compared to prior art foams. In another of its aspects, the present invention relates to a process for producing such an isocyanate-based polymer foam.
2. Description of the Prior Art
Isocyanate-based foams, such as polyurethane foams, are known in the art. Generally, those of skill in the art understand isocyanate-based polymers to be polyurethanes, polyureas, polyisocyanurates and mixtures thereof.
It is also known in the art to produce foamed isocyanate-based polymers. Indeed, one of the advantages of isocyanate-based polymers compared to other polymer systems is that polymerization and foaming can occur in situ. This results in the ability to mould the polymer while it is forming and expanding.
One of the conventional ways to produce a polyurethane foam is known as the “one-shot” technique. In this technique, the isocyanate, a suitable polyol, a catalyst, water (which acts as a reactive “blowing” agent and can optionally be supplemented with one or more physical blowing agents) and other additives are mixed together at once using, for example, impingement mixing (e.g., high pressure). Generally, if one were to produce a polyurea, the polyol would be replaced with a suitable polyamine. A polyisocyanurate may result from cyclotrimerization of the isocyanate component. Urethane modified polyureas or polyisocyanurates are known in the art. In either scenario, the reactants would be intimately mixed very quickly using a suitable mixing technique.
Another technique for producing foamed isocyanate-based polymers is known as the “prepolymer” technique. In this technique, a prepolymer is produced by reacting polyol and isocyanate (in the case of a polyurethane) in an inert atmosphere to form a liquid polymer terminated with reactive groups (e.g., isocyanate moieties and active hydrogen moieties). To produce the foamed polymer, the prepolymer is thoroughly mixed with a lower molecular weight polyol (in the case of producing a polyurethane) or a polyamine (in the case of producing a modified polyurea) in the presence of a curing agent and other additives, as needed.
Regardless of the technique used, one of the characteristics of isocyanate-based foams, particularly polyurethane foams produced from formulations containing aromatic isocyanates (e.g., TDI, MDI and mixtures thereof), is their tendency to change colour after production. Specifically, polyurethane foams are known to change colour from slightly off-white directly after production to mustard yellow within hours or days after production.
The colour change usually occurs on the surface of the foam within a relatively thin layer as a result of exposure to light (fluorescent or non-fluorescent) and atmosphere. This type of environmental induced colour change is different from scorching discoloration resulting from thermal induced degradation and does not have any significant impact on the physical properties of the foam. Nevertheless, the colour is unwelcome by most customers.
It is known in the prior art that isocyanate-based foams produced from formulations containing aliphatic and/or alicyclic-based isocyanates are less susceptible to environmentally induced discoloration when compared to isocyanate-based foams produced from formulations containing aromatic-based isocyanates. See for example any one of the following: Japanese patent application S52-52997/1977, Japanese patent application H02-255817/1990, Japanese patent application H04-318016/1992 and U.S. Pat. No. 4,542,166 for teachings of yellowing-resistant isocyanate-based foams produced from formulations containing aliphatic and/or alicyclic-based isocyanates. Unfortunately, isocyanate-based foams produced from formulations containing aliphatic and/or alicyclic-based isocyanates have been met with very limited commercial success as a result of relatively poor physical properties such as melting upon exposure to ultraviolet radiation, relatively low heat resistance, relatively high material costs (i.e., of the isocyanates) and the like.
To date the prior art has not developed a technique for slowing down or avoiding the post-production colour change in isocyanate-based foams such as polyurethane foams produced from formulations containing aromatic-based isocyanates.
Accordingly, there remains a need in the art an isocyanate-based polymer foam, particularly such foams produced from formulations containing aromatic-based isocyanates having improved anti-yellowing properties. Ideally, such a foam could be produced without the need to alter foam moulding equipment and other chemical components in the formulation used to produce the foam. Thus, it would be highly desirable to be able to produce such a foam by the addition to the formulation of a relatively low cost additive have no significant deleterious effect on the resultant foam